1. Field of the Invention
The invention relates to an apparatus and method for level clamping a composite video signal, and more particularly, to an apparatus and related method for level clamping the composite video signal utilizing a dithering circuit.
2. Description of the Prior Art
Since the introduction of the Advanced Television Systems Committee (ATSC) standard, great effort has been expended to perfect a digital TV system. For compatibility, a digital TV system designed for decoding a digital TV signal that is in compliance with an ATSC specification must also be capable of decoding a composite video signal that is in compliance with specifications of traditional analog TV systems, such as the National Television System Committee (NTSC) standard or the Phase Alternate Line (PAL) standard. In this way, a user is able to watch programs using the same equipment regardless of the signal being transmitted by a digital TV signal or an analog TV signal.
Before any signal processing is performed on the composite video signal, a blank level of the composite video signal must be clamped in advance. Since the amplitude and the phase of the composite video signal are crucial to decoding the composite video signal, how accurately the blank level is clamped can seriously affect the performance of a video decoder for decoding the composite video signal. Conventionally, if one needs to clamp the blank level of the composite video signal more accurately, the clamping precision of a level clamping circuit must be improved so that an output signal of the level clamping circuit for clamping the blank level of the composite video signal is finer. The level clamping circuit is generally integrated into the video decoder and is implemented in the digital domain for clamping the desired blank level. In other words, for the blank level of the composite video signal to be more accurate, a step size of the output of the level clamping circuit must be smaller, which increases the cost and the size of the level clamping circuit.